1. Field of the Invention
The present invention relates generally to coaxialcable splicing and more particularly to a method and a means for obtaining a highly reliable water-seal for a Teflon-jacketed cable splice.
2. Description of the Prior Art
It has been determined that prior art potting materials and techniques used for the sealing of coaxial cables are unable to consistently supply the requisite water sealing under high pressures and over large temperature ranges when the cables have flexible outer jackets. Such prior-art methods generally comprise only a single step of potting. For example, a typical splicing technique would comprise the following steps. The center conductors of the coaxial cables to be spliced would be soldered together and then covered with a shrink tube set to overlap the adjacent dielectric layers of the cables. Then the shields of both cables would be joined either by stretching them over the shrink tubing and soldering or by installing a short piece of braided shield between the existing shields and soldering. Next, the flexible jackets (typically made of a fluoro-polymeric material) of the cables would be prepared for potting by etching their surfaces with special caustic compounds commercially available for this purpose. After these preparatory steps, the potting compound would then be mixed and applied to the entire splice area. This area would include an overlap region for the jackets of both of the cables. Initially the liquid potting compound would be contained in its position surrounding the splice by an appropriate holder until it cures.
Typical potting compounds utilized for cablesplicing become quite hard when completely cured. For example, Scotchcast 8, a frequently used potting compound, has a hardness of 70 on the Shore D hardness scale. When fluoro-polymer (Teflon) jacketed cables with such a splice are bent and flexed during handling, the hard edge of the cured potting cylinder tends to cut the jacket of the softer Teflon at its emergence point from the potting.
In addition, the coefficient of expansion for potting compounds such as Scotchcast 8 is 40% higher than the coefficient for Teflon. Thus temperature variations in the cable environment produce surface stress at the bond interface which tend to break the water seal. This is especially so if the pressure is high.
Finally, when water does get into the shield region (via Teflon breaks at the Teflon-potting interface), it can easily work its way along the shield down to the region where the shrink tubing is sealing the center conductor. Since this is not necessarily a watertight seal, the water could leak under the shrink tubing and form a low-resistance path between the center conductor and the shield, thus compromising the performance of the cable.